Vacuum operated work holding devices are commonly employed as workpiece gripping elements to engage and transport a workpiece in a manufacturing operation, to load and unload sheet metal parts into and from a die, or to carry a part, such as an automobile windshield, to the vehicle in which it is to be installed. Such vacuum operated work holding devices employ a control apparatus which uses a venturi passage in a body which is connected to a source of air under pressure. Air flow through the venturi passage induces a sub-atmospheric pressure in the throat of the venturi and in a passage connecting the venturi throat to the interior of the vacuum cup. This sub-atmospheric pressure will induce a vacuum within the cup when the cup engages a workpiece surface.
Prior control devices and vacuum cups included mechanical means for maintaining the vacuum in the cup. Further, mechanical means were used to inject air under pressure into the vacuum cup to rapidly release the vacuum between the vacuum cup and the workpiece. These mechanical vacuum and blow-off means are described in U.S. Pat. Application Ser. No. 07/645,135 filed Jan. 24, 1991 and U.S. Pat. Application Ser. No. 07/647,803 filed Jan. 30, 1991. Vacuum cup assemblies, such as the above-referenced patent applications, include a control apparatus which is attached directly to the vacuum cup itself. Although such an assembly is advantageous where singular vacuum cups are needed to transport a workpiece, such an assembly becomes burdensome where a plurality of vacuum cups must be simultaneously controlled to transport a workpiece. A further disadvantage of these and other vacuum cup devices is the inability to increase the vacuum level or flow rate created by the apparatus itself. Thus, where a greater force is necessary to aid in transporting a heavy workpiece, additional vacuum cups, including their respective control devices, must be added to the material handling system.